tests-windows: Make flow extractor work on Windows

tests-windows: Make flow extractor work on Windows

+Traceback (most recent call last):
+  File "../.././flowgen.py", line 204, in <module>
+    flows = os.fdopen(3, 'wb')
+OSError: [Errno 9] Bad file descriptor
./library.at:4: exit code was 1, expected 0

Unfortunately 3/4/5/.. FDs don't work on Windows. Switch to a filename open
instead of opening the FDs.

Signed-off-by: Alin Gabriel Serdean <aserdean@ovn.org>
Acked-by: Ben Pfaff <blp at ovn.org>